The present invention relates to liquid ring vacuum pumps, and more particularly, to varying the flow rate and the volume of a service liquid in the stages of a two stage liquid ring vacuum pump.
Liquid ring vacuum pumps are well known and widely used in industrial applications for a smooth nonpulsating gas or vapor removal. The working parts of a liquid ring vacuum pump include a multi-bladed impeller eccentrically mounted in a cylindrical casing which is partially filled with a service liquid. As the impeller rotates, the liquid is thrown by centrifugal force to form a liquid ring which is concentric with the periphery of the casing. Due to the eccentric positioning of the impeller relative to the casing and the liquid ring, the spaces or cells between adjacent impeller blades are cyclically filled and emptied with the service liquid as the impeller rotates. During rotation of the impeller any air or gas trapped in the cell is compressed and discharged from the casing through an outlet port leaving the cell available to receive air or gas which is presented to the suction port of the casing.
Two stage liquid ring vacuum pumps are used in applications requiring relatively high vacuums. The two stage liquid ring vacuum pump can produce and efficiently maintain suction pressures from 150 to 25 mm Hg absolute. A two stage liquid ring vacuum pump has two impellers working in series on a volumetric ratio which can be as high as 3:1, first stage to second stage. The impellers are affixed to a common shaft and are rotated at the same rotational speed. The two stages can accommodate a greater capacity at lower absolute suction pressures than a single stage of compression. However, two stage liquid ring vacuum pumps exhibit the inherent problem of lower capacity at high absolute suction pressures.
To maximize efficiency, the desired operating parameters and required system component sizes must be matched for a given two stage liquid ring vacuum pump. However, the relatively large range of operating parameters forces trade offs in matching component capabilities. That is, some individual components may not be maximized in view of trade offs with other components at certain operating conditions.
Two stage liquid ring vacuum pumps are well suited for efficiently and reliably maintaining the low absolute pressure at the holding point. However, the two stage liquid ring vacuum pumps require a substantial period of time to completely evacuate a vessel. In fact, evacuation with a two stage liquid ring vacuum pump is usually slower than other available methods. For example, evacuation ejectors or single stage vacuum pumps are often more effective than two stage liquid ring vacuum pumps during the evacuation stage.
Therefore, the need exists for maximizing the capacity of two stage liquid ring vacuum pumps. A need also exists for improving the evacuation capacities of the liquid ring vacuum pump, without sacrificing capability elsewhere. In addition, the need exists for optimizing horsepower requirements throughout the range of operating parameters. The need also exists for improving the evacuation capacity of a two stage liquid ring vacuum pump without requiring excessive recirculating and cooling facilities for the service liquid.